No human or robot will ever know what plugins you're using unless it's referred on the source code or you have directory browsing enabled ...lol not make it look like autoblog...
do you think google spider is so stupid and can not crawl your wp- directories and see
and when you your wp directories hidden (hacker see a blank page) but not the google crawler.
They see what plugins you have installed on your site.
the blank page on WP directory is just empty index.php ,what do you mean by robots can tell what plugins being installed by visiting empty index.php?
Even if they let say know, so what? You're not breaking any law